The concept of adding odorants to consumer gases in accordance with the aforegoing, so as to indicate the leakage of poisonous or explosive gases for instance, has long been known to the art. One example of gases which may be odorized in this way is oxygen, which if leaking to the surroundings can result in extremely serious accidents caused by fire or explosion. Other examples include combustible gases, such as natural gas, propane, butane, town gas, etc., which can also cause serious accidents in the form of fire and explosions. Since the majority of odorous additives, such as tetrahydro thiophene, butyl mercaptan, dimethyl sulphide, etc., are readily ignitable substances which require the application of special techniques when added to oxygen for instance.
Finish Patent Application 870146 discloses a method of adding an odorant to oxygen, in which a concentrated gas, so-called master gas, is produced in a separate chamber or space by adding to pure oxygen gas an odorant in a concentration of 1,000-10,000 ppm. This concentrated master gas is added to the consumer gas in a separate chamber, or space, in an amount such that the odorant will be present in the consumer gas in a concentration of 5-50 ppm.
When the master gas contains solely oxygen and odorant, for instance dimethyl sulphide, problems can occur, however, when filling the master gas containers. For instance, when filling the containers, it is impossible to avoid passing through a concentration range in which the mixture is combustible, at least in a part of the container. There is thus a risk of the mixture igniting and exploding.
One method of avoiding this risk is disclosed in the Finnish Patent Application No. 872278. This application describes a method of producing a concentrated master gas comprising oxygen and an odorant, such as dimethyl sulphide. According to this method, the master gas container is first filled with a mixture of dimethyl sulphide and nitrogen or helium gas. The concentration of dimethyl sulphide lies within a range of 0.5-2.5%. Pure oxygen gas is then added until the desired working pressure in the container is reached, for instance a pressure of 200 bars.
One drawback with the master gas produced in accordance with the aforedescribed methods, however, is that the master gas must not be subjected to temperatures which are so low as to cause the odorant to condense, for instance during transportation and storage. Once being condensed, it takes a very long time for the dimethyl sulphide to return to its gaseous state.
Prior publications DE-B-1185330 and WO 91/17817 describes methods which reduce this problem in that the odorant is dissolved in a gas which exists in liquid phase at room temperature and under pressure. Propane, butane, carbon dioxide, sulphur hexafluoride and nitrous oxide have been given as examples of suitable gases in this respect. These gases also fulfil the requirement of not having a negative influence, in the majority of cases, on the process in which the odorized gas is used.
It is suggested in prior publication DE-B-1 185 330 that the odorized master gas is taken from the pressure vessel and delivered to the consumer gas conduit via a fine setting valve which can normally be maintained at a predetermined setting during the consumption of all of the master gas. However, in the case of large variations in the flow rate of the consumer gas, it is said that the flow rate of the master gas can be controlled in response to such variations.
In practice, however, this and other known solutions do not provide the odorant metering accuracy that is desired. This is because the odorant vehicle gas has a much higher vapor pressure than the liquid odorant. Thus, the gas volume present above the liquid phase of the master gas in the pressure vessel will consist essentially of vaporized vehicle gas and only a very small part of vaporized odorant liquid. As the volume of the liquid phase in the pressure vessel diminishes when master gas is delivered to the consumer gas, the increasing volume of vaporized vehicle gas in the pressure vessel will result in an increase in the relative concentration of the liquid odorant in the liquid phase in the pressure vessel.